Cell Biology & Macromolecules

Molecular architecture of $\text{Na}^+/\text{K}^+$-ATPase (sodium pump):

• alpha and 3 beta units is an incorrect subunit ratio.
• The native, physiologically active oligomeric complex of $\text{Na}^+/\text{K}^+$-ATPase in the plasma membrane is organized as an $(\alpha\beta)_2$ heterotetramer comprising two catalytic $\alpha$ subunits (approx. $110\text{ kDa}$ each, harboring ATP binding, phosphorylation, and ion-binding sites) and two glycosylated $\beta$ subunits (approx. $55\text{ kDa}$ each, essential for correct folding and membrane targeting), often accompanied by regulatory FXYD ($\gamma$) peptides.
• alpha and 2 beta units does not represent functional assembly.
• While the minimal catalytic functional unit is an $\alpha\beta$ heterodimer, the native holoenzyme dimerizes into an $\alpha_2\beta_2$ complex.

Methods for analyzing membrane architecture:

• Patch clamp technique measures electrical ion currents passing through individual membrane ion channels.
• FRAP (Fluorescence Recovery After Photobleaching) measures the lateral diffusion rate and mobility of membrane proteins or lipids, not their static physical distribution.
• Freeze-etching involves controlled sublimation of ice from the fractured surface to expose peripheral surface features.
• Freeze-fracture electron microscopy cleaves rapidly frozen biological membranes along the hydrophobic interior plane between the two lipid monolayers, revealing intramembrane protein particles (IMPs) embedded in the P-face and E-face, directly visualizing protein distribution and providing foundational proof for the Singer-Nicolson fluid mosaic model.

Coordinated organelles in secretory protein biosynthesis:

• Ribosomes perform mRNA decoding and polypeptide chain elongation.
• Rough endoplasmic reticulum (RER) mediates co-translational translocation, initial protein folding, disulfide bond formation, and core N-glycosylation.
• Lysosomes are acidic catabolic organelles that degrade macromolecules, not organelles for protein synthesis or maturation.
• Golgi apparatus performs post-translational modifications (O-glycosylation, phosphorylation, oligosaccharide trimming) and proteolytic maturation.